10 Gigabit Ethernet is an increasingly popular communication standard, with a nominal data rate of 10 Gbit/s. One form of 10 Gigabit Ethernet is IEEE 10GBASE-T, used to provide 10 gigabit per second connections over unshielded or shielded twisted pair copper wires. Multiple pairs of the wires are provided that define multiple physical channels. Each channel serves as a bi-directional signaling link that simultaneously routs data in both transmit and receive directions. For a given data transmission, the receiver end of the link that receives the transmission generally needs to cancel its own transmit signal back the other way, or the signals become superimposed on each other leading to faulty data reception. The circuit that carries out this transmit cancellation is generally known as a transmit hybrid circuit. Applications such as 10GBASE-T often desire 60 dB of cancellation by the transmit hybrid circuit.
FIG. 1 illustrates a conventional transmit hybrid circuit 100 for high-speed Ethernet applications. The circuit includes a transceiver 102 that includes a transmitter 104 and a receiver 106. The transmitter includes a first voltage mode digital-to-analog converter DAC1 that receives a digital data input DATA, and generates an analog voltage output across a transmit impedance network RTx. The output couples to an input/output (I/O) node 110. A second digital-to-analog converter DAC2 also receives the digital data input and generates a mirror copy of the analog output voltage across a hybrid impedance network RHr. The mirror voltage output is fed to one input of the receiver 106 and coupled to a receiver impedance network RTr, while the other output of the receiver couples to the I/O node 110. The I/O node 110 connects to a bidirectional link 114 that has an opposite end connected to a link partner transceiver (not shown).
While the conventional transmit hybrid circuit described above works well for its intended applications, the dual DAC configuration presents power and accuracy problems. While multiple DACs provide a certain level of programmable flexibility, the power dissipation may be unacceptable for certain low-power environments. Further, the transmit output voltage and the hybrid voltage need to be matched within a certain level of precision in order to cancel the effects of the superimposed transmit signal within the desired 60 dB level. This is very difficult to do with multiple DACs, and the result often includes a nonlinear voltage mismatch.
What is needed is a more power-efficient and accurate way to carry out transmit hybrid functions in high-speed Ethernet circuits.